1. Field of the Invention
The invention relates to handovers in mobile communication systems. Particularly, the invention relates to a method for performing handover between a Wireless Local Area Network (WLAN) and a licensed band cellular communication system.
2. Description of the Related Art
Recently Wireless Local Area Networks (WLAN) have become important in mobile communications. The advantage of WLANs over licensed band cellular communication systems such as the Universal Mobile Telecommunication system (UMTS) and Global System of Mobile communications (GSM) lies in the facts that they use an unlicensed band and the cell sizes are much smaller. Hereinafter, the term cellular communication system refers to a communication system operating in the licensed band. These facts make possible to build private WLANs operated by small corporate entities and individual users. The cost of wireless communication in these WLANs is significantly cheaper than in cellular systems. WLANs have mostly been used for Internet access, but the idea of providing voice communications over WLANs has recently gained momentum. In order to obtain a wide market share for voice over WLAN technologies and to provide a reliable service experience for end-users, it is necessary to be able to provide dual system terminals, which support both WLAN and licensed band based radio access. In other words, it must be possible for users to roam in both WLANs and cellular communication systems.
Usually WLAN radio access is used in urban areas where there exists a WLAN infrastructure, whereas cellular communication systems are used in areas outside the WLAN coverage.
3G Partnership Project has standardized the IP Multimedia Subsystem (IMS) in order to cater for VoIP and other IP based multimedia services. Typically, a UMTS Radio Access Network is used to access a core network, which supports the IMS. However, existing circuit switched core network infrastructures, which comprise Mobile Switching Centers (MSC), Home Location Registers (HLR), Visitor Location Centers (VLR), Camel Service Entities (CSE) and Service Control Points (SCP), provide a wide range of services. Operators wish to accommodate dual system terminals with both WLAN and licensed band radio access capabilities. In order to provide seamless service for the users of dual system terminals, it is beneficial if there were a mechanism to provide handover between the two radio access technologies. Especially, it is important to support handovers from WLAN to a cellular communication system. This is due to the fact that WLANs offer a much more limited and patched radio coverage, which is usually even overlapping with the radio access provided by the cellular communication system. Because of this, for the users there always exists the option of falling back to the use of the cellular communication system. The users may start their sessions in the WLAN side and fall back to the use of the cellular radio access, if the WLAN signal strength shows signs of weakening.
One possible solution for implementing a handover from a WLAN to a cellular communication system is proposed by the company Cingular Wireless, Atlanta, USA. In the solution proposed by Cingular, the voice bearers for a dual-system mobile terminal are routed by the IP Multimedia Subsystem (IMS) to a Media Gateway (MGW) and from the MGW further to a Gateway Mobile Services switching Center (GMSC), which routes the call to the destination. At the time of handover a telephone number is allocated by the IMS and provided to the mobile terminal. The telephone number refers to a conference bridge in association with the MGW. Thereupon, the mobile terminal de-registers from the IMS and performs a GSM location updating to the GMSC. In order to keep the voice bearer, mobile terminal establishes a circuit switched call to the conference bridge. The call set-up requested is routed to the conference bridge by the GMSC using the telephone number. The problem associated with the mentioned solution for handovers is that it requires the use of a conference bridge. The calls from a dual-system mobile terminal must always be routed via a MGW and a GMSC to a called party in order to enable the mobile terminal to reach the MGW using a normal outgoing circuit switched call at the time of handover. This represents a solution, which introduces complication to the call routing procedures. A further disadvantage is the implementation of handover procedures in mobile terminal side, which deviate from the normal intra-cellular communication system handover procedures. The mobile terminal is not able to reuse the software for the implementing of intra-cellular communication system handovers in the implementing of inter-system handovers. Instead, in the Cingular system unusual handover processing logic must be implemented to the mobile terminal software.
Reference is now made to FIG. 1, which illustrates a communication system supporting dual system terminals in prior art. In FIG. 1 there is shown a dual-system mobile station 100. Mobile station 100 is capable of communicating both using an unlicensed band radio access and using licensed band radio access. There is an unlicensed band Base Transceiver Station (BTS) 102. BTS 102 is connected to a Customer Premises Equipment (CPE) 104, which is, for example, an access router. To CPE 104 is connected a Session Border Controller (SBC) 106. There may be a number of unlicensed band base transceiver stations, which are connected via CPE 106 to SBC 106. SBC 106 acts as a SIP proxy and hides the address space within the operator's network, which comprises at least IP access network 108 and the IMS network elements, from MS 100. User plane traffic to/from MS 100 goes via SBC 106. Session border controller 106 is connected to IP access network 108, which performs the packet transport for all user plane related data traffic. In FIG. 1 there is also shown a licensed band radio access network 120 to which is connected a base transceiver station 121. Licensed band radio access network 120 is, for example, a 2G GSM radio access network or a 3G UMTS radio access network. A licensed band IP Connectivity Access Network (IP-CAN) functionality connected to access network 120 comprises at least a serving GPRS support node SGSN 122 and a gateway GPRS support node 124. SGSN 122 performs all mobility management related tasks and communicates with a Home Subscriber Server (HSS) 150. GGSN 124 provides GPRS access points to a media gateway 126 and to a Proxy Call State Control Function (P-CSCF). GGSN 124 establishes Packet Data Protocol (PDP) contexts, which are control records associated with a mobile subscriber such as mobile station 100. A PDP context provides an IP address for packets received from mobile station 100 or any other mobile station that is connected to the licensed band IP connectivity access network 120. The GPRS is disclosed in the 3G Partnership Project specification 23.060.
The communication system illustrated in FIG. 1 comprises IP Multimedia Subsystem (IMS) functionality. The network elements supporting IMS comprise at least one Proxy Call State Control Function (P-CSCF), at least one Inquiring Call State Control Function (I-CSCF), at least one Serving Call State Control Function S-CSCF, at least one Brakeout Gateway Control Function (BGCF) and at least one Media Gateway Control Function (MGCF). As part of the IMS there is also at least one Home Subscriber Server (HSS) Optionally, there is also at least one Application Server, which provides a variety of value-added services for mobile subscribers served by the IP multimedia subsystem (IMS). The IMS is disclosed in the 3G Partnership Project (3GPP) specification 23.228. P-CSCF 130 receives signaling plane packets from GGSN 124. P-CSCF approves Quality of Service (QoS) allocation for the signaling plane PDP context opened in GGSN 124. In the signaling plane packet is carried a Session Initiation Protocol (SIP) signaling message. The Session Initiation Protocol (SIP) is disclosed in the Internet Engineering Task Force (IETF) document RFC 3261. The signaling message is processed by P-CSCF 130, which determines the correct serving network for the mobile station that has sent the signaling packet. The determination of the correct serving network is based on a home domain name provided from mobile station 100. Based on the home domain name is determined the correct I-CSCF, which in FIG. 1 is I-CSCF 132. I-CSCF 132 hides the topology of the serving network from the networks, in which mobile station 100 happens to be roaming. I-CSCF 132 takes contact to home subscriber server 150, which returns the S-CSCF name, which is used to determine the address of the S-CSCF to which the mobile station 100 is to be registered.
In FIG. 1 the S-CSCF determined for mobile station 100 is S-CSCF 134. S-CSCF 134 obtains information pertaining to mobile station 100 from HSS 150. The information returned from HSS 150 may comprise trigger information that is used as criterion for notifying an application server 152. Application server 152 may be notified on events relating to incoming registrations or incoming session initiations. Application server 152 communicates with S-CSCF 134 using the ISC-interface. The acronym ISC stands for IP multimedia subsystem Service Control interface. The ISC interface is disclosed in the 3GPP specification 23.228. The protocol used on ISC interface is SIP. AS 152 may alter SIP invite message contents that it receives from S-CSCF 134. The modified SIP invite message is returned back to S-CSCF 134. If the session to be initiated is targeted to a PSTN subscriber, the SIP invite message is forwarded to a BGCF 140. BGCF 140 determines the network in which PSTN interworking should be performed. In case PSTN interworking is to be performed in the current network, the SIP invite message is forwarded to MGCF 142 from BGCF 140. MGCF 142 communicates with MGW 126. The user plane packets carrying a media bearer or a number of interrelated media bearers for the session are routed from GGSN 124 to MGW 126 as illustrated in FIG. 1 using line 162.
In case mobile station 100 communicates via the unlicensed band radio access the packets are sent via BTS 102, CPE 104 and SBC 106 to IP access network 108. Signaling packets are received in P-CSCF 138. Based on a home domain name provided in the signaling packet P-CSCF 138 determines the correct I-CSCF, to which the signaling packet is to be sent. In FIG. 1 the I-CSCF is I-CSCF 136. I-CSCF 136 inquires the HSS 150 in order to determine the correct S-CSCF for mobile station 100. In this case S-CSCF 134 is determined. Depending on the called party SIP URI S-CSCF 134 determines whether the session is to be routed to a second S-CSCF or to a BGCF such as BGCF 140.